Virtual function call in derived class constructor & destructor

Question

Virtual function call in derived class constructor & destructor

question 20

https://learn.microsoft.com/cpp/cppcx/ref-classes-and-structs-c-cx?view=msvc-170

I saw the following in this link.

The behavior of trying to access a member of a class that has already allowed the destructor to run is not defined. There is a high possibility that the operation of the program will be interrupted.

I want to call a virtual function that handles a member within a derivative class constructor, a destructor. Even if I pay attention to the initialization order (by initializing members after the virtual function call is complete), is there a possibility that an undefined action will occur because the destructor has already been executed? Or is it okay to say that it's safe if I just pay attention to the order?

Minxin Yu 13,501 Reputation points Microsoft External Staff

2023-11-27T08:38:16.2+00:00

What kind of C++ project are you using? Can you provide a minimal reproducible example?

question 20

This is my sample code.

//main.cpp
#pragma once
#include "Base.h"
#include "Derived.h"

int main()
{
    Derived TestObj;
    return 0;
}

//Base.h
#pragma once
#include <iostream>
using namespace std;

class Base
{
    public:
      Base()
      {
        cout << "Base : constructor" << endl;
        log();
      }
 
      virtual ~Base()
      {
          cout<<"Base : destructor" << endl;   
      }

      virtual void log() const = 0;
};

//Base.cpp
#include "Base.h"
void Base::log() const
{
    cout << "Base : log" << endl;
}

//Derived.h
#pragma once
#include <vector>
#include "Base.h"
using namespace std;

class Derived : public Base
{
  private:
      int* Arr = new int[1];

  public:
      Derived()
      {
          cout << "Derived : constructor" << endl;
          log(); //In experiment1 (When I did not paid attention to the initialization order)
          Arr[0] = 10000;
          //log(); //In experiment2 (When I paid attention to the initialization order)
      }
    
      ~Derived()
      {
          cout << "Derived : destructor" << endl;
          //log(); //In experiment3 (When I paid attention to the initialization order)
          delete[] Arr;
          //log(); //In experiment4 (When I did not paid attention to the initialization order)
      }
      virtual void log() const override;
};

//Derived.cpp
#include "Derived.h"

void Derived::log() const
{
    cout << "Derived : log (" << Arr[0] << ")" << endl;
}

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Minxin Yu 13,501 Reputation points Microsoft External Staff

2023-11-27T08:38:16.2+00:00

What kind of C++ project are you using? Can you provide a minimal reproducible example?
question 20 Reputation points

2023-11-27T09:31:42.2266667+00:00

This is my sample code.

//main.cpp #pragma once #include "Base.h" #include "Derived.h" int main() { Derived TestObj; return 0; }

//Base.h #pragma once #include <iostream> using namespace std; class Base { public: Base() { cout << "Base : constructor" << endl; log(); } virtual ~Base() { cout<<"Base : destructor" << endl; } virtual void log() const = 0; };

//Base.cpp #include "Base.h" void Base::log() const { cout << "Base : log" << endl; }

//Derived.h #pragma once #include <vector> #include "Base.h" using namespace std; class Derived : public Base { private: int* Arr = new int[1]; public: Derived() { cout << "Derived : constructor" << endl; log(); //In experiment1 (When I did not paid attention to the initialization order) Arr[0] = 10000; //log(); //In experiment2 (When I paid attention to the initialization order) } ~Derived() { cout << "Derived : destructor" << endl; //log(); //In experiment3 (When I paid attention to the initialization order) delete[] Arr; //log(); //In experiment4 (When I did not paid attention to the initialization order) } virtual void log() const override; };

//Derived.cpp #include "Derived.h" void Derived::log() const { cout << "Derived : log (" << Arr[0] << ")" << endl; }

Answer 1

Minxin Yu 13,501 Microsoft External Staff

Hi, @question

delete Arr; It is recommended to use delete[] Arr;

log(); //In experiment1 and //log(); //In experiment4

The variable has not been assigned a value or has been deleted.

Reading an uninitialized value results in undefined behavior.

Best regards,

Minxin Yu

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question 20 Reputation points

2023-11-28T02:03:48.1733333+00:00

However, experiments 1 and 4 were intended to make me behave badly. I want to know if the function should not be called after the destructor call according to the document, even if I pay attention to the initialization order like Experiments 2 and 3.
Minxin Yu 13,501 Reputation points Microsoft External Staff

2023-11-28T03:20:45.5533333+00:00

In this simple example, pay attention to the initialization order and the program will work as expected. But it is better not to call virtual functions in constructors and destructors. It is considered dangerous most of the times and must be avoided whenever possible.
question 20 Reputation points

2023-11-28T09:00:14.21+00:00

I understand. Thank you for your response!

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Virtual function call in derived class constructor & destructor

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